Rattles can be hard to locate.
Understatement of the year/decade/millennium !!!!!!!!!!!
(off topic) I have a 15" Trace Elliot Combo, it was fine when I got it but soon it developed a buzz. It drove me crazy. Eventually I tracked it down to a handle that was spring loaded, the spring had broken.
(on topic) I have just pulled the trigger on a 3015LF and have a mid driver (the number escapes me now) on order to build either a fEARful 15+6 or 15+6tube. Cannot decide which, it depends which fits in my car best. I am not renowned for rushing projects so don't hold your collective breaths.

